Each aircraft is provided with a flight command system connecting the piloting units (stick, rudder bar) with the aerodynamic control surfaces (ailerons, spoilers, vertical rudder, pitch motivator) of the aircraft. Those systems have been developed over the time with the technical progress from purely mechanical non-assisted systems to electrical (command) systems being hydraulically assisted.
On the first systems, the effort exerted by the pilot on the piloting units was integrally transmitted to the rudders and corresponded to the aerodynamic efforts. In order to minimize the muscular efforts, compensators or trims have been created to be able to vary the control surface position corresponding to a nil muscular effort from the pilot.
Over the technical evolutions, the trims have been essentially kept for operational needs (from the pilots). As on the present rudder control systems, there is no more mechanical connection between the rudder bar and the vertical rudder, the trim only serves to vary the nil effort position of the rudder bar.
Usually, a rudder control system for an aircraft, in particular for an airplane, of the electrical type, comprises:                a rudder bar adapted to be operated by a pilot;        means for detecting the position of the rudder bar and generating an electrical control signal being representative of such position; and        a flight control calculator receiving such electrical control signal and generating, with the help of the latter, control orders being transmitted to actuating means for a vertical rudder of the aircraft.        
The vertical rudder is so called electrical, since the order controlled by the pilot (position of the rudder bar) is transmitted to the flight control calculator under the shape of an electrical signal. The calculator uses then such pilot order to determine, thru internal control laws, the control order being effectively applied to the vertical rudder.
Despite such electrical characteristics, the rudder bar remains a complex mechanical unit. It especially comprises a spring assembly being able to create again an artificial sensation (as the rudder bar is not mechanically connected with the control surface of the aircraft anymore). Such artificial sensation generating means also comprise a mechanical rudder compensation or trim function. Such a function being implemented by a mechanical compensator or trim allows the pilot to control the nil effort rudder bar position. The control is carried out by the pilot thru buttons located in the cockpit.
Such mechanical rudder trim meets several operational needs:                in the case of an engine failure.        The failure of an engine of the aircraft generates a yawing moment. Thus, in order to keep an optimum flight line, the pilot must control a rudder bar order so that the vertical rudder generates a corrective yawing moment being opposed to the moment related to the engine failure.        The constant application of such order obliges the pilot to exert a constant muscular effort on the rudder bar. In order to enable the pilot to release the pedals while keeping his position (and thus his order), he calls on the mechanical rudder trim function.        The use of such function allows the nil effort position (initially supposed to be the neutral position) to be brought up to the current position of the rudder bar (required by the pilot). Thus, the pilot may release the rudder bar while keeping the current position; and                    in the case of an airplane being twisted or asymmetrically loaded (fuel, passengers).                        In those cases, the pilot can be in a position to ask for a constant slight rudder order so as to cancel or to compensate for the dissymmetry (optimum sideslip, minimum rudder clearance). To do so, the pilot may call on the mechanical rudder trim function. Thus, the rudder bar can take a non nil order without the pilot operates on the pedals (by a slight displacement of the nil effort point).        
The presence, on an electrical rudder control system such as described above, of a mechanical rudder trim meeting the above mentioned needs, has some disadvantages, in particular in terms of mass, cost, installation, congestion, maintenance, overall reliability, etc.